filter-repo: throw an error if user specifies any path starting with a slash

All paths are intended to be relative paths, relative to the project
root, not to the filesystem root.  There have been a few people who
didn't understand this, and then ended up with fast-import crashes that
are not very clear.  Check for it early and throw a simple error message
instead.

Signed-off-by: Elijah Newren <newren@gmail.com>

git-filter-repo

@@ -1632,11 +1632,15 @@ class FilteringOptions(object):
           raise SystemExit(_("Error: With --path-rename, if OLD_NAME and "
                              "NEW_NAME are both non-empty and either ends "
                              "with a slash then both must."))
+        if any(v.startswith(b'/') for v in values):
+          raise SystemExit(_("Error: Pathnames cannot begin with a '/'"))
         components = values[0].split(b'/') + values[1].split(b'/')
       else:
         mod_type = 'filter'
         match_type = suffix
         components = values.split(b'/')
+        if values.startswith(b'/'):
+          raise SystemExit(_("Error: Pathnames cannot begin with a '/'"))
       for illegal_path in [b'.', b'..', b'.git']:
         if illegal_path in components:
           raise SystemExit(_("Error: Invalid path component '%s' found in '%s'")

t/t9390-filter-repo.sh

@@ -1108,8 +1108,22 @@ test_expect_success 'other startup error cases and requests for help' '
 		test_i18ngrep "could not parse.*3GiB" err &&
 
 		test_must_fail git filter-repo --path-rename foo/bar:. 2>err &&
-		test_i18ngrep "Invalid path component .\.. found in .foo/bar:\." err
+		test_i18ngrep "Invalid path component .\.. found in .foo/bar:\." err &&
 
+		test_must_fail git filter-repo --path /foo/bar 2>err &&
+		test_i18ngrep "Pathnames cannot begin with a ./" err &&
+
+		test_must_fail git filter-repo --path-rename foo:/bar 2>err &&
+		test_i18ngrep "Pathnames cannot begin with a ./" err &&
+
+		test_must_fail git filter-repo --path-rename /foo:bar 2>err &&
+		test_i18ngrep "Pathnames cannot begin with a ./" err &&
+
+		test_must_fail git filter-repo --subdirectory-filter /foo 2>err &&
+		test_i18ngrep "Pathnames cannot begin with a ./" err &&
+
+		test_must_fail git filter-repo --subdirectory-filter /foo 2>err &&
+		test_i18ngrep "Pathnames cannot begin with a ./" err
 	)
 '